Research On The Electromigration Phenomena Of Lead-Free Solder And Intermetallic Cu₆Sn₅ Phase In Lap Solder Joints

Electromigration of solder joint is becoming one of the key factors of the reliability of electronic packaging. For the requirements of environmental protection, there is an inevitable trend of lead-free solder taking place of lead solder. Although academics have taken a lot of researches on the electromigration of lead-free solder joints, the characteristics of migration behaviors and mechanism of lead-free solder joints have not be further studied yet. So this research used Sn3.0Ag0.5Cu and Sn0.7Cu to make lap joints as the research object, and studied on the distribution of current density and the rules of grain orientation affects the distribution of atoms in the lap joints by non-situ and in-situ electromigration experiments combine with computer numerical simulations, EBSD and EDS.According to the results of computer numerical simulations. We confirmed that there are current crowding areas in the pad tip and cutting-edge inflection point of pad end, for the current minimum path is here and the dramatic changes in the shape of conductor.There were reduction of the cathode pad, IMC polarization effects and electromigration cumulative effects in lap joints in the non-situ electromigration experiments. We studied on the grain orientation and the atoms distribution in Sn3.0Ag0.5Cu and Sn0.7Cu lap joints and found the facts that Cu atoms diffuse fast along the c-axis of theβ-Sn crystal and grain boundary. And we used this theory to explain the problems of the different growth orientations of IMC, anode IMCs'abnormal aggregation, violations of the electromigration cumulative effect in IMC thickness and the different migration paths of Cu atoms in electromigration which many other traditional electromigration theory can not explain.In the in-situ electromigration experiment, we found the phenomena of remelting in joint. And the melting solder joint have the phenomena of IMC polarization and reduction of the cathode pad. However, there was no electromigration phenomenon in the joint which have a good heat dissipation. That indicate that electromigration is a combined effect of electricity and heat. Thermal conditions play a key role in electromigration failure. We also found SnCu in-situ samples had stress pattern on its cross-section. Because there are lower resistances along the a, b axes ofβ-Sn grains, the rotation of grain orientation will make grains'a, b axes parallel to the current direction and that will reduce the resistance of solder joints. Stress pattern is the representation of the compression between the grains by grains rotation.Comparing the electromigration characteristics of SnCu with SnAgCu. The grains in SnCu solder joints are much smaller than SnAgCu. So the atoms'diffusion and migration in SnCu will show strong isotropic. Therefore SnCu is more reliable than SnAgCu in electromigration.